The invention concerns a method for the chemical reduction, consisting of one or more stages of reaction, of powdered metal compounds as the material to be reduced, to powdered metal or to powdered metal compounds at a lower oxidation state than the predetermined final product, in a temperature controlled reaction chamber with input and output openings for the material to be reduced, for the gaseous reducing medium and, alternatively, for the carrier gas. The method is particularly suitable for the reduction of refractory powdered metal oxide into powdered metal, for example, the reduction of en oxide to tungsten, or even of molybdenum oxide to molybdenum.
Until now, the methods of powder metallurgy have dominated the manufacture of refractory metals and their further processing. As a result, the quality of the powdered metal, particularly the purity and the grain structure, has a decisive influence on the quality of the powder-metallurgical products produced from it.
The specialist has therefore always ascribed great importance to the production steps of metal extraction, which produce pure powdered metal directly. This especially concerns the reduction of powdered metal oxides of refractory metals, for example W.sub.4 O.sub.11 or MoO.sub.3 to pure metallic powder in one or more stages of reduction.
It is therefore noteworthy, that these oxides have predominantly been reduced in pusher-type furnaces for decades, for reasons unchanged up to the present. For this processing, the powdered oxides are smeared into a thin layer in tablet-like ceramics or metal seers and directed (pushed) step by step, at an established rate for several hours through a high temple pusher-type furnaces with zones of different temperature and different atmospheric conditions. As a rule, hydrogen gas is introduced as the reducing agent. The processing times and therefor the reduction times are around 10.sup.3 -2.times.10.sup.4 seconds. The specific throughput, measured as tons of material to be produced per unit volume (m.sup.3 ) of the reaction chamber and hour, is approximately 0.04 for pusher-type furnaces.
Another known method for reducing these types of metallic compounds which is seldom used in practice, is the so led rotary kiln method Here the charging stock of the material to be reduced moves in a rotary kiln tilted slimly from the horizontal. The processing times are comparable to that of pusher-type kilns. Here, adequate furnace dwelling or reduction times are likewise in the range of 10.sup.3 -10.sup.4 seconds and the specific throughput is approximately 0.055 tons per cubic meter and hour.
Both of these methods have a disadvantage which has been known for a long time, namely the small specific throughput of the powder to be reduced, relative to the size of the installation. The source of the small specific throughput is the restricted access of the reducing gas to the powder surface of the material to be reduced.
To overcome the discussed disadvantages of rotary kilns and pusher-type kilns, it has been proposed to undertake the reduction in a fluidized bed oven In this method, the metal compound to be reduced is brought as powdered charging stock into a temperature controlled reaction chamber on a metal grating, By the reducing gas, mixed with a carrier gas if necessary, flows from below through the material to be reduced, which is positioned on the grating, thereby stirring up the material, bringing it into a quasi-fluid gas particle phase, and reducing it to powdered metal in contact with the same.
Yet even in this method, the dwelling time of the material to be reduced in the reaction chamber for adequate homogeneous reduction of the particles is 10.sup. -10.sup.4 seconds. Accordingly, the specific throughput of the material to be reduced is around one power of ten above that of pusher-type and rotary kilns.
The sources of these unsatisfactorily long dwelling or reaction times are the partial reoxidation of already reduced particles in the charging stock, and the unavoidable formation of individual time-varying "channels" in the bulk material through which the gas flows. The materials in the zones bordering these channels is only partially, if at all, supplied with reducing gas and reduced.
The publication by A. V. Savin "On the practice and theory of reduction of tungsten oxides", Izvestiya Rossuskoi Akademii Nauk. Metally, No. 4, pages 22-26, 1993, English version appearing under the publication number ISSN: 0568-5303, published in 1993 by Allerton Press. Inc., 150 Fifth Ave., New York, N.Y. 10011; deals with the metallurgical events and with the kinetics of the reduction of tungsten oxide. In this context it gives practical laboratory experiments to help with the reduction of powdered tungsten oxide. For this purpose, loose tungsten oxide powder was introduced from above into a vertical tube, 250-300 mm long, heated to 1,200.degree. C., over a small hopper, and traveled through this tube in free fall within 0.25 seconds. There was a reducing gas in the tube whose influence on the passage time of the powder through the tube could be disregarded. The reduction of tungsten oxide occurred at a moderate rate, so that the process had to be repeated several times with the same powder, in order to obtain reaction data which appeared to support the model over the individual reaction events. In an alternative experimental arrangement, a film of WO.sub.3 was placed on a wire which was brought into a reducing atmosphere, and heated for approximately 1 second by a current passage. This resulted in a reduction of the oxide that escaped precise quantification.
In the professional world, the concept of "cyclone" has evolved for a reaction chamber, externally similar to the fluidized-bed chamber, for carrying out reactions of chemical materials. In contrast to the procedure in a fluidized-bed chamber, the solid, gaseous, and occasionally even liquid materials which are provided for a reaction in a cyclone, often premixed and swirled many times, individually or together, are continuously fed into the upper area of the reaction chamber with mass throughput, as a whole vertical, or blown in a prescribed direction. The materials introduced in this way move under the effects of the gravitational and centrifugal forces as well as the flow conditions of the gases in the reaction space. They react in the desired way and manner, and exit the chamber, mainly at the end of the reaction chamber opposite the entrance, as solid, liquid, or gaseous end products. Gaseous end products are released at the reaction chamber's upper end, from case to case, depending on the flow conditions.
The origination of a flow field of the reaction material with axial, radial, and high tangential velocities is common to all cyclone-like reaction chambers. As a rule, very large relative velocities between solids and gases appear. Large velocity gradients produce high intensities of turbulence, thereby effecting high rates of thermal and material diffusion.
These types of cyclone reactors are useful for the pyrolysis of sawdust: J. Lede, F. Verzaro, B. Antoine, J. Villermaux, "Flash Pyrolysis of Wood in a Cyclone Reactor", Chem. Eng. Proc. 20 (1986), pages 309-317; J. Cousins, W. Robinson, "Gasification of Sawdust in an air blown cyclone Gasifier", Ind. Eng. Chem. Process Des. Dev. 24 (1985), pages 1281-1287; or even for the burning of slag and sewage sludge: T. Murakami, et al, "Characteristics of Melting Process for Sewage Sludge", Wat. Sci, Tech. 23 (1991), pages 2019-2028.
Finally, cyclones are also described for exothermal metallurgical processes, e.g. for melting copper and lead concentrations, and copper concentrations containing zinc (DE 33 41 154 Al; A Lange, "Das Schwebeschmelzen und andere leistungsintensive Prozesse" [Flash-smelting and other Production Intensive Processes], Erzmetall 13 (1960), pages 151-159).
It is nevertheless common to all metallurgical processes conducted in cyclones, that the reaction materials are introduced as gaseous and solid particles, and the reduced metals leave the cyclone molten, i.e. they are collected as smelt in a settling kiln at the reaction chamber's lower outlet.
The shape and chemical structure of all materials participating in the reaction are determining factors for the constructive arrangements actually tried, within broad bounds, and the process parameters used in the cyclone in practice.
A reaction chamber corresponding to the U.S. Pat. No. 4,555,387 is used for a procedural step for man, molybdenum trioxide from a slag containing molybdenum sulphide. It corresponds to a cyclone insofar as powdered molybdenum sulphide is introduced into a reaction chamber from above together with a reaction gas, and these pass through on a prescribed path. Slags containing molybdenum di-sulphide are added to the reaction chamber separately in particle form together with oxygen as the reaction gas and with additional carrier gases. There, they are brought to the reaction temperature in the so-called flame front. The end products leaving the reaction space are volatile molybdenum trioxide, liquid slag, and residual gas.